The present invention pertains to high voltage coaxial feedthrough bushings, and in particular to such bushings having complicated geometrical shapes. The bushing of this invention is directed to radio frequency (RF) electrical systems operated at frequencies of 300 megahertz or less, voltages of 100 KV or less, and power levels of at least 1 megawatt.
One application for such bushings is in the Princeton Large Torus Ion Cyclotron Resonant Frequency analysis being conducted at the Princeton Plasma Physics Laboratory. These experiments are directed to a particular method of heating a magnetically confined plasma, by launching RF fields into the plasma. Once the RF fields are launched into the plasma, energetic charged particles that will interact with the tokamak walls can be excited. Surface physics problems associated with RF heating of tokamak plasma can be classified into material interaction with RF fields in plasma particles. To maximize the power coupling of the plasma, both the voltage standoff and current carrying capacity of the RF components need to be optimized. The power transmission of the Princeton Large Torus Ion Cyclotron Resonant Frequency experiments is presently limited by voltage breakdown in the vacuum feed through bushing.
Conventional bushings employ a ceramic or other dielectric structural member which maintains the alignment of inner and outer coaxial conductors. The dielectric mass to be employed within the bushing should be reduced so as to minimize charging current, but must not be reduced to the point where electrical and mechanical strength of the bushing is degraded. Further improvements in the dielectric member can be realized if the path length of the outer surface of the internal insulator member is maximized without hampering the control of uniform electrical stress appearing across the dielectric surface.
In addition, if the bushing having the aforementioned advantages is to comprise in an upgraded replacement in an existing installation, it must provide higher voltage and power levels while meeting the length and girth restrictions imposed by a particular installation.
Electrical feedthrough bushings of the type to which the invention is directed must provide a barrier between a gas-filled environment and a hard vacuum, while transmitting high levels of voltage, frequency and power into the electrical load. For the purpose of the invention, a "hard" vacuum is defined as being at least as great as 10.sup.-6 Torr.
Vacuum integrity during operation of the bushings used in the Laboratory must be maintained to a very high degree at all times, since impurity influx during operation of a magnetic confinement device would severely degrade, if not destroy operation thereof, requiring protracted down-time for the extraction of the impurity.
The high voltage electrical art has long recognized the difficulties in determining electric field stress in various dielectric materials arranged in complex geometrical forms. However, such complex forms offer the possibility of enhanced bushing performance if the proper interaction between electrically conductive and dielectric components of the bushing could be achieved.
It is therefore an object of the present invention to provide a compact high voltage RF bushing having increased voltage standoff.
Another object of the present invention is to provide a compact high voltage RF bushing having increased power handling capability.
Yet another object of the present invention is to provide a high power RF bushing which provides control of the internal electric field within the bushing so as to substantially eliminate regions of increased electrical stress.
Another object of the present invention is to provide, within a gas-filled bushing having an inner and an outer electrical conductor separated by a dielectric member, a maximized path length along the surface of the dielectric member so that the product of the surface gradient and the ceramic dielectric constant is less than the breakdown strength of the dielectric gas during normal operation.
Yet another object of the present invention is to provide a high voltage RF bushing in which the electric field therewithin is controlled to obtain a uniform stress across the dielectric surface, while controlling the ratio of the outer and inner diameters of the dielectric member to be the electrical constant "e" where possible to minimize surface gradients.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.